The invention relates to configuring devices that are coupled to a communications channel such as a bus.
A typical computer includes one or more buses over which devices can communicate with each other. A primary bus such as the Peripheral Component Interconnect (PCI) bus may interconnect several integrated devices, such as processor/memory subsystems, bridge controllers, peripheral controllers, and peripheral add-in boards.
Another bus in the system may be a point-to-point bus in which active communication may occur only between two devices residing on the bus. One such bus is the Accelerated Graphics Port (AGP) bus, as defined by the Accelerated Graphics Port Interface Specification, Revision 2.0, published in May 1998. The AGP bus provides a relatively high-speed communications path between a graphics device and main memory, which may be utilized to store some video data (including 3D graphics data).
In the AGP architecture, an AGP bus is defined that is coupled to a target and a master. The target typically includes an AGP interface in a host bridge, and the master typically includes a graphics accelerator. The AGP bus defines a point-to-point interface in which active communication occurs only between two AGP agents. In some implementations, more than two devices may be attached to the bus, in which two AGP masters and an AGP target reside on the AGP bus. In this configuration, one of the two masters is disabled so that only one active master and one active target are present on the bus. Because of signal quality issues when more than two AGP devices are coupled to the AGP bus, however, a need exists to ensure proper operation of the AGP bus when more than two devices are coupled to the bus.
In general, in one embodiment, a method of configuring devices coupled to a communications channel in a system includes accessing storage elements in the devices that contain values representative of communications characteristics of the devices. The values in the storage elements are updated to indicate that certain communications characteristics are unavailable.
In general, in another embodiment, a system includes a communications channel and devices coupled to the communications channel. Each device contains an indication of a communications characteristic of the device. A control entity configures each device by changing the indication to indicate a different communications characteristic.
Other embodiments and features will become apparent from the following description and from the claims.